(1) Field of the Invention
The invention relates to an improved method of making an active matrix liquid crystal (AMLC) display panel, and in particular to a method of manufacturing an Liquid Crystal Display (LCD) panel that more effectively and efficiently eliminates or reduces the effect of electrostatic damage during processing steps.
(2) Description of Prior Art
Modern flat panels LC displays have found wide usage, particularly in small portable computers capable of operating on batteries. Such displays in general have a layer of liquid crystal material, that is capable of changing its optical characteristics when exposed to an electric field. The LC material is usually sandwiched between two transparent or semitransparent panels. One of the panels has a matrix consisting of a set of spaced parallel conductive scan lines, and a second set of orthogonal spaced parallel conductive data lines. The areas defined by two adjacent scan lines and two adjacent data each contains a transparent electrode that define a pixel. An array of electronic switch elements usually thin film transistors, associated with the scan and data lines, operate to selectively apply an electrical potential to the electrodes. The applied potential influences the liquid crystal material in the immediate area. The potential is applied across a small electrode on one substrate to a common conductive electrode on the opposed panel. FIG. 1 depicts a schematic diagram of a single cell of a LC display panel. A field effect transistor (FET) 10, normally a very small thin film transistor having an amorphous or polycrystalline silicon layer, has the gate connected to a scan line 12, and the source joined to a data line 14. The drain is connected to the transparent pixel electrode which is the upper plate of the capacitor 16. The hatched area of the capacitor 16 designates the liquid crystal material between the panels, and the lower plate of the liquid crystal capacitor 16 designates the opposed common electrode on the opposed panel. Storage capacitor 18, which is connected in parallel with the capacitor 16 stores a potential applied to the liquid crystal layer. Capacitor 18 may be built into the lower panel. This capacitor may be omitted. In operation, the transistor 10, in response to signals on the scan and data lines, is selectively turned on to influence the liquid crystal material and thereby selectively produce an opaque area. The panel is normally back lighted. An image can thus be formed on the display panel in response to signals applied to the scan and data lines through contact pads (not shown) associated with the lines.
In FIG. 2 there is illustrated a sequence of process steps for fabricating a TFT panel, known to the prior art. As indicated in blocks 11 and 13 a first substrate, normally of glass, that is either transparent or translucent, is prepared and an active matrix array that includes the scan and data lines, contact pads, thin film transistors, transparent electrodes, capacitors, etc. are fabricated. Then a second transparent or translucent glass substrate, as indicated by blocks 15, 17 and 19 are prepared with a common electrode and possibly color filters as is known in the art. Blocks 21, 23, 25, 27, 29 and 31 indicate known process steps which will be described in more detail later in this description.
In performing the indicated process steps for assembling the LCD panel, very troublesome problems are caused by the generation of static electricity. Electrostatic charges are developed when any two substances (solid, liquid or gas) make contact and are then separated. These electrostatic charges can be great enough to damage or cause the malfunction of electronic parts, assemblies and equipment during their discharge to ground or to another object. During the manufacturing of TFT liquid crystal flat panel displays, electrostatic energy may be generated by many of the process steps, for example the spin drying process for the TFT array and the alignment treatment for the LCD process. During the production of thin film transistor array, the spin drying process is necessary to dry the glass substrate after every wet process, the glass substrates are loaded into a spin roller and rotate the roller with the substrates, the friction between glass substrate and the air generate the electrostatic charges. Alignment treatment 21 for LCD fabrication process also generates electrostatic charges. A thin alignment layer such as polyimide is coated on the glass, and the surface is rubbed with something, usually like a soft cloth, in the same direction as the liquid crystal molecules will be aligned. Thus the ESD (electrostatic discharge) protection is necessary before finishing the whole TFT and LCD fabrication processes.
Even a small amount of electrostatic energy can severely damage the dielectric films of very small transistor elements and cross-over regions of matrix lines. It is known to provide a conductive line surrounding the matrix that is joined to each of the contact pads connected to the scan and data lines. This line must later be removed before the display panel is tested and put into operation. The shorting of the data pads places the elements at the same potential and eliminates or minimizes damage due to electrostatic discharge. However, since the production yield of the active matrix array is quite low, the arrays must be tested to separate the defective one before they are processed further. The shorting line prevents the electrical testing of the array. If it is removed for the testing, it leaves the substrate with its array vulnerable for the later process steps which generate static electricity.
The shorting ring is most conveniently formed when operations for producing the matrix array is formed, which is before the tests are applied. Thus, it would be advantageous to form the shorting ring as the matrix array is formed and to render the shorting ring operative and non-operative at various stages of manufacture and testing of the panel display. In the prior art this concept is not known.